1. Field of the Invention
The present invention relates to lithographic systems, and more particularly to interferometric lithography.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which is commonly referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., photoresist) provided on the substrate.
Instead of a circuit pattern, the patterning means can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Resolution achieved by the overall semiconductor manufacturing process depends not only on the optics involved, but also on the chemical processes employed (e.g., interactions between the photoresist and the etching chemicals, etc.).
When resolutions reach nanometer scale, such as 30-100 nm, it is very difficult to use conventional masks, reticles, and patterning arrays to accomplish this. The current resolution limit of lens-based systems is approximately 45 nm.
Interferometric lithography tools have been proposed, either within an immersion system or by themselves, to form small nanometer scale features. These typically use a Talbot interferometer scheme. In order to achieve higher resolutions, non-symmetrical Talbot interferometer schemes have been suggested. However, it is sometimes very difficult to achieve a desirable fringe contrast across a large image field when using these systems because of path length differences within the interfering beams.
Therefore, what is needed is an interferometric lithography system and method that generates a desired contrast across an entire field at resolution dimensions matching or surpassing current conventional, lens based lithography system capabilities.